A known example of a practical piezoelectric material is lead zirconate titanate (PbZrxTi(1-x)O3, (wherein x≈0.5)), which is a solid solution of lead zirconate (PbZrO3) and lead titanate (PbTiO3).
Lead zirconate titanate cannot be obtained in the form of a single crystal having a size, for example, a cross-sectional area of 1 cm2, suitable for practical use and therefore is used in the form of a sintered body which is a cluster of fine crystals. Therefore, there is a problem in that only an isotropic property that is the average of piezoelectric properties of the fine crystals, which form the sintered body, in orientations can be used because a crystal orientation exhibiting excellent piezoelectric properties of a single crystal of lead zirconate titanate cannot be selectively used.
A complete solid solution type piezoelectric single crystal (relaxor-lead titanium solid solution) that contains a relaxor having the composition Pb(A1, A2, . . . , B1, B2, . . . )O3 and lead titanate, which has the composition PbTiO3, and that has a complex perovskite structure can be grown to a size sufficient to obtain a tabular piezoelectric single crystal device having a size, for example, a cross-sectional area of 1 cm2 or more, suitable for practical use. Therefore, a piezoelectric device having a practical size can be manufactured such that a crystal orientation exhibiting excellent piezoelectric properties of a material is selected depending on applications.
An example of the complete solid solution type piezoelectric single crystal, which contains a relaxor having the composition Pb(A1, A2, . . . , B1, B2, . . . )O3 and lead titanate, which has the composition PbTiO3, is a lead magnesium niobate (Pb(Mg, Nb)O3)-lead titanate (PbTiO3) solid solution (hereinafter simply referred to as PMN-PT or PMNT).
The piezoelectric single crystal can be grown by the Bridgman method or Czochralski method, in which a single crystal is grown from a source melt; a flux method in which crystals are precipitated from a source solution; a top seeded solution growth (TSSG) method which is a type of flux method; or another method.
In the case of growing the piezoelectric single crystal by using the above methods, after a crystal source such as a powder mixture of the relaxor and lead titanate or a polycrystalline sintered pellet of a relaxor-lead titanate solid solution is put in a crucible and then melted or dissolved in a solvent, the resulting crystal source is gradually solidified in one direction using a seed crystal or the seed crystal is pulled out of the resulting crystal source, whereby the piezoelectric single crystal is desirably grown. The piezoelectric single crystal grown as described above is in the form of an ingot. The unprocessed piezoelectric single crystal is hereinafter referred to as a piezoelectric single crystal ingot.
In the case of producing the piezoelectric single crystal ingot by one of the above methods, there is a problem in that the compositional fraction (mole percent) of lead titanate varies monotonically in the growth direction of a single crystal. In the case of growing, for example, the PMN-PT, the compositional fraction of lead titanate in the piezoelectric single crystal ingot increases monotonically in the growth direction of the single crystal.
A piezoelectric single crystal device manufactured from the piezoelectric single crystal ingot is known to have piezoelectric properties, such as a Curie temperature (Tc), a dielectric constant (∈), an electromechanical coupling coefficient (k), and a piezoelectric constant (d), variable depending on the concentration of lead titanate in the piezoelectric single crystal device.
A portion of the piezoelectric single crystal ingot has a lead titanate concentration unsuitable for practically useful piezoelectric properties and heat resistance because of the variation of the compositional fraction of lead titanate. This causes a reduction in yield.
It could therefore be helpful to provide a complete solid solution type piezoelectric single crystal ingot in which the compositional fraction of lead titanate is inhibited from varying in the growth direction thereof, to provide a process for producing the complete solid solution type piezoelectric single crystal ingot, and to provide a piezoelectric single crystal device which can be manufactured from the complete solid solution type piezoelectric single crystal ingot at low cost and which has uniform piezoelectric properties.